Op-amp
An operational amplifier (often op-amp or opamp) is a high- electronic voltage with a and, usually, a output. Op-amps are among the most widely used electronic devices today. The amplifier's differential inputs consist of a non-inverting input (+) with voltage V''+ and an inverting input (–) with voltage ''V-. The op-amp amplifies the difference in voltage between the two, which is called the differential input voltage. The output voltage of the op-amp V''out is given by the equation :: V_\text{out} = A_\text{OL} (V_+ - V_-), :where ::''A''OL = gain of the amplifier. The magnitude of ''A''OL is typically very large (100,000 or more for integrated circuit op-amps), and therefore even a quite small difference between ''V+ and V''- drives the amplifier output nearly to the supply voltage. Situations in which the output voltage is equal to or greater than the supply voltage are referred to as ''saturation of the amplifier. The magnitude of A''OL is not well controlled by the manufacturing process, and so it is impractical to use an open-loop amplifier as a stand-alone . Feedback The popularity of the op-amp as a building block in s is due to its versatility. By using , the characteristics of an op-amp circuit, its gain, input and , etc. are determined by external components and have little dependence on s or in the op-amp itself. High input impedance at the input terminals and low output impedance at the output terminal(s) are particularly useful features of an op-amp. In the non-inverting amplifier on the right, the presence of negative feedback via the ''R''f, ''R''g determines the ''closed-loop gain A''CL = ''V''out / ''V''in. Equilibrium will be established when ''V''out is just sufficient to "reach around and pull" the inverting input to the same voltage as ''V''in. The voltage gain of the entire circuit is thus 1 + ''R''f/''R''g. As a simple example, if ''V''in = 1 V and Rf = ''R''g, ''V''out will be 2 V, exactly the amount required to keep ''V- at 1 V. Because of the feedback provided by the R''f, ''R''g network, this is a ''closed-loop circuit. Log amplifier A log amplifier is an for which the output voltage V''out is ''K times the of the input voltage V''in. This can be expressed as, :: V_\text{out} = K \ln\left(\frac{V_\text{in}}{V_\text{ref}}\right) :where ::''V''ref = in volts ::''K = scale factor. The logarithm amplifier gives an output voltage which is proportional to the logarithm of applied input voltage. To design a logarithm amplifier circuit, high performance s like LM1458, LM771, LM714 are commonly used and a compensated logarithm amplifier may include more than one. Logarithmic amplifiers are used in many ways, such as: # To perform mathematical operations like multiplication, division and exponentiation. Multiplication is also sometimes called mixing. This is similar to operation of a , and is used in s, audio synthesis methods, and some measurement instruments (i.e. power as multiplication of current and voltage). # To calculate the dB value of a given quantity. # As a . # Extending dynamic range of other circuits, like of transmit power in circuits, or s. The relationship between the input voltage V_\text{in} and the output voltage V_\text{out} is given by: :: V_{\text{out}} = -V_\text{T} \ln \left(\frac{V_\text{in}}{I_\text{S} \, R} \right) :where :: I_\text{S} = saturation current of the diode. :: V_\text{T} = of the diode. References Category:Electronics